The promise is trustworthy because God has proven Himself to be trustworthy.
Adam and Eve wanted to stay put and make a city of the garden, but God wouldn't have it that way. For Abraham, God's refrain: "Go out from your country, and from your family, and from your father's house to a land that I will show you." For Joseph, his movement out and away from his family took the form of Arab traders, accusations of rape, and nocturnal visions in the way of the Lord who bends all of creation towards the final, ultimate goal of sending a preacher to announce absolution to Joseph's brothers. The promise, not obedience to a reasonable course of action, is the thing for God.
Our Lord will build His salvation promise on rebellion, false worship, and scandal. He will complete His work, not when and where His people would have it, but where He will raise up a Savior for them. It will be an absurd mystery that runs through all of history, built on promise, mercy, and sacramental grace done in and through a chosen people called "the church", that stretches from Adam to Paul and on through all God's chosen people, through His saints, to the Last Day.
The church is made to bear God's promise in the mystery of our Lord's coming to be born, His suffering and death, and His having been raised from the dead. The church is not a nodding heads club of religious hyperactives. They are not a holiness society for pious oddballs. The church is God's instrument, marked in their flesh with the Name which is above all names. The Name for which every knee in heaven and on earth and under the earth shall bow in worship. Jesus' Name carries the promise to its completion in and through His chosen people, in simple earthly words, water, bread and wine. These are the tokens of the promise our Lord has declared to us. The concrete Gospel. The marks of the church.
God's Promise
But the promise is an absurd thing to those looking for a reasonable demand of obedience. God's promise that is born, lives, suffers, and is executed on Golgotha is absurd to those who want to settle down and make a city of a garden. In the same way, we do not require tent or tabernacle because we have a Lord who tabernacles amongst us and we are, flesh and bone, body and blood, His temple. Our wandering is halted. We do not have to seek the promised land, the promised land is delivered to us. The new Jerusalem is embodied by God's promise, Jesus Christ. Wherever Jesus is, the promise is delivered, the Gospel is given in immediate, tangible, effective words, and the church is established by the blood of Israel Himself, the Suffering Servant.
The promise is trustworthy because God has proven Himself to be trustworthy
By grace through faith we are drawn into our Lord's promise, and all creation (as witnessed by Jonah's experience) is bent toward our absolution in Christ's Name. This is the ultimate absurdity, that God gives the promise away to His betrayers, to those who run away from Him, and to those who are lost in doubt as to whether they are truly His chosen people. But, in Christ it is all true. The promise is trustworthy because God has proven Himself to be trustworthy in that He has brought us a preacher who points us to the font and to His table and to the Good News that we are His bride, and He has dressed in robes of righteousness for His Names' sake. He prepares us for the wedding feast to come when we will be brought into the new Jerusalem through twelve gates, built upon twelve foundations, shepherding us along a street of pure gold into the divine banquet hall.
And the sign that all this is happening now in the present tense is the blood of the Lamb. He is our new Jerusalem. Jesus is our New Testament. He is our second Adam that redeems humanity from the power of sin and death and hell. Jesus Christ is the promise's critical mass Who carries us along in His promised grace and mercy today and always, world without end.
